Fix debug pages

[ryanhamley]

Launch Checklist

• briefly describe the changes in this PR
  • The changes to the debug page in Fix resource loading edge cases #8612 broke the debug pages when served locally via the yarn start-debug command. This is the only configuration I've found to allow the debug pages to work properly when served locally and when tested with a file:// URL
• manually test the debug page

[mourner]

Debug pages through yarn run start-debug work fine for me on master — I wonder why they're broken on your side.

[ryanhamley]

Well that's weird. I always see http://localhost:9966/access_token_generated.js net::ERR_ABORTED 404 and the page fails to load. I've tried on Node 8 and 10 just to rule out that possibility and I tried in multiple browsers.

[ryanhamley]

Ah so, I have always just gone to localhost:9966/debug in my browser. That doesn't work anymore. The URL has to conclude in / or the path to an html file. So localhost:9966/debug/ works fine. My mistake.